Ekaterina Kurae, most commonly referred to as Katja, is a young Russian girl and a central figure in the narrative. Despite her age of approximately ten years, she holds the formidable title of the Qwaser of Copper, wielding power over that element. She is a transfer student to St. Mihailov Academy, having been sent from Russia at the behest of Miyuri Tsujidou, who sought a foreign companion to balance the social dynamics at the school.

On the surface, Katja presents the image of a shy, innocent, and childlike doll. She is fully aware of her delicate, fairy-tale beauty and cultivates this persona meticulously, often referring to herself in the third person to enhance her perceived cuteness amongst her classmates and teachers, who regard her with a mix of admiration and awe. This facade, however, conceals her true nature. In reality, Katja is domineering, arrogant, prideful, and possesses a malicious, almost regal sense of entitlement. She uses her innocent exterior as a tool to manipulate and seduce other young women, particularly those susceptible to her charms, to emotionally and physically dominate them. Her motivations are largely driven by a desire for control and a steady supply of Soma, the sacred substance that fuels a Qwaser's power, which manifests as breast milk. She is deeply prideful and harbors a strong dislike for being treated as a child or being saved by her rival, the Iron Qwaser known as Sasha.

Katja’s role in the story is that of a powerful, independent operator. Though initially aligned with the Athos faction, she primarily acts in her own self-interest. Her background hints at a deep, mysterious connection to the late Tsar Nicholas II and the Romanov imperial family of Russia, with implications that she may be connected to or even be the Tsarina Alexandra herself, though this remains a layered aspect of her character. This connection is further emphasized by her most notable ability and companion: a grim, archaic gynoid named Anastasia, whom Katja calls mother. Due to her own physical frailty as a young girl, Katja rarely fights directly. Instead, she uses her copper-based powers to control Anastasia with the precision of a puppeteer. The gynoid is capable of channeling high-voltage electricity, restraining multiple attackers, creating copper objects from nothing, and engaging in combat at a level impossible for a human. Beyond controlling her guardian, Katja can manipulate copper wiring and redirect electrical currents to electrocute and incinerate her foes.

Regarding her key relationships, Katja maintains a harem of Marias, young women who serve as her source of Soma. Her primary and most significant relationship is with Hana Katsuragi, who she refers to as a Magdalene, keeping her as a personal slave and a living reserve of energy. She later comes to dominate Lulu Shiizaki and Ayame Satsuki, who also fall under her control and serve her needs. Despite her cruel and dismissive behavior towards them, she exhibits a possessive and deeply caring attitude towards her gynoid, Anastasia, showing genuine grief when the construct is destroyed and giving it a proper burial. While she initially holds the school in contempt, she eventually begins to open up to her peers, particularly Miyuri Tsujidou, whom she comes to regard as an older sister figure. Over the course of the narrative, despite her pride and insistence on controlling everything, she shows signs of developing genuine bonds, even as she continues to exert her will over her devoted followers.